System for optical wireless power supply

ABSTRACT

A system for optical wireless power transmission to a power receiving apparatus generally situated in a mobile electronic device. The transmitter has an optical resonator with end reflectors and a gain medium positioned between them, such that an optical beam is generated. The frequency of the beam is selected such that it is absorbed by almost all transparent organic materials in general use. A beam steering unit on the transmitter can direct the beam in any of a plurality of directions, and the beam is absorbed on the receiver by means of an optical-to-electrical power converter, through a low reflection surface. The band gap of this power converter is selected to be smaller than that of the gain medium. The receiver has a voltage converter including an inductor, an energy storage device and a switch. A beam steerer controller ensures that the beam impinges on the receiver.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/811,260 filed Jul. 28, 2015 which claims the benefit of U.S.Provisional Application No. 62/193,368 filed Jul. 16, 2015. Theseapplications are herein incorporated by reference in its entirety forall purposes.

FIELD OF THE INVENTION

The present invention relates to the field of wireless power beaming,especially as applied to use of a laser based transmission system tobeam optical power in a domestic environment to a mobile electronicdevice.

BACKGROUND

There exists a long felt need for the transmission of power to a remotelocation without the need for a physical wire connection. This need hasbecome important in the last few decades, with the popularization ofportable electronic devices operated by batteries, which need rechargingperiodically. Such mobile applications include mobile phones, laptops,cars, toys, wearable devices and hearing aids. Presently, the capacityof state of the art batteries and the typical battery use of a smartphone intensively used may be such that the battery may need chargingmore than once a day, such that the need for remote wireless batteryrecharging is important.

Battery technology has a long history, and is still developing. In 1748Benjamin Franklin described the first battery made of Leyden jars, thefirst electrical power source, which resembled a cannon battery (hencethe name battery). Later in 1800, Volta invented the copper zincbattery, which was significantly more portable. The first rechargeablebattery, the lead acid battery, was invented in 1859 by Gaston Plante.Since then the energy density of rechargeable batteries has increasedless than 8 times, as observed in FIG. 1, which shows the energydensity, both in weight and volume parameters, of various rechargeablebattery chemistries, from the original lead acid chemistry to thepresent day lithium based chemistries and the zinc-air chemistry. At thesame time the power consumed by portable electronic/electrical deviceshas reached a point where several full battery charges may need to bereplenished each day.

Almost a century after the invention of the battery, in the periodbetween 1870 and 1910, Tesla attempted the transmission of power overdistance using electromagnetic waves. Since then, many attempts havebeen made to transmit power safely to remote locations, which can becharacterized as over a distance significantly larger than thetransmitting or receiving device. This ranges from NASA, who conductedthe SHARP (Stationary High Altitude Relay Platform) project in the 1980sto Marin Soljacic, who experimented with Tesla-like systems in 2007.

Yet, to date, only three commercially available technologies allowtransfer of power to mobile devices safely without wires namely:

-   -   Magnetic induction—which is typically limited in range to just a        few mm;    -   Photovoltaic cells—which cannot produce more than 0.1 Watt for        the size relevant to mobile phones when illuminated by either        solar light or by available levels of artificial lighting in a        normally (safe) lit room; and    -   Energy harvesting techniques—which convert RF waves into usable        energy, but cannot operate with more than 0.01 W in any        currently practical situation, since RF signal transmission is        limited due to health and FCC regulations.

At the same time, the typical battery of a portable electronic devicehas a capacity of between 1 and 100 Watt*hour, and typically requires adaily charge, hence a much higher power transfer at a much longer rangeis needed.

There is therefore an unmet need to transfer electrical power, over arange larger than a few meters, safely, to portable electronic devices,which are typically equipped with a rechargeable battery.

A few attempts to transfer power in residential environments, usingcollimated or essentially collimated, electromagnetic waves, have beenattempted. However, commercial availability of such products to the massmarket is limited at the current time. A few problems need to be solvedbefore such a commercial system can be launched:

-   -   A system should be developed which is safe.    -   A system should be developed which is cost effective.    -   A system should be developed which is capable of enduring the        hazards of a common household environment, including        contamination such as dust and fingerprints or liquid spills,        vibrations, blocking of the beam, unprofessional installation,        and periodic dropping onto the floor.

Currently allowed public exposure to transmitted laser power levels areinsufficient for providing useful amount of power without a complexsafety system. For example, in the US, the Code of Federal Regulations,title 21, volume 8, (21 CFR §8), revised on April 2014, Chapter I,Subchapter J part 1040 deals with performance standards for lightemitting products, including laser products. For wavelengths outside ofthe visible range, there exist, class I, class III-b and class IV lasers(class II, IIa, and IIIa are for lasers between 400 nm and 710 nm, e.g.visible lasers). Of the lasers outside the visible range, class 1 isconsidered safe for general public use and classes IIIb and IV areconsidered unsafe.

Reference is now made to FIG. 2 which is a graph showing the MPE(maximal permissible exposure value) for a 7 mm. pupil diameter, forclass I lasers, according to the above referenced 21 CFR §8, for 0.1-60seconds exposure. It can be seen from the above graph that:

-   -   (i) The maximum permissible exposure levels generally (but not        always) increase with wavelength, and    -   (ii) Even if the laser is turned off some 0.1 second after a        person enters the beam, in order to meet the requirement        specified in 21 CFR §8, no more than 1.25 W of light can be        transmitted, and that at wavelengths longer than 2.5μ, with the        limit orders of magnitude less at shorter wavelengths.

Thus, without some kind of safety system, only a few milliwatts of laserpower are allowed to be transmitted, which even if completely convertedback to electricity, would supply significantly less power than thepower needed to charge most portable electronic devices. A cellularphone, for example, requires from 1 to 12 W for charging, depending onthe model.

To transmit power higher than that of class 1 laser MPE, a safety systemis needed. None, to the best of the applicants' knowledge, has yet beencommercialized for transmitting significant power levels in residentialenvironment accessible to untrained people.

Building of a robust safety system is difficult. It is well known in theart that fingerprints and dust scatter laser light and that transparentsurfaces reflect or scatter it. If high power is to be transferred, thena class IV (or IIIb) laser would be needed, which would require areliable safety system. For Class IV lasers, even scattered radiationfrom the main beam is dangerous. According to the 21 CFR §8, as revisedon April 2014, Chapter I, Subchapter J part 1040, lasers emittingbetween 400 nm and 1400 nm, having more than 0.5 W beam output, areusually considered class IV lasers for exposures above 0.5 sec, and evenscattered radiation from such lasers may be dangerous. Such lasers arerequired to have a lock key and a warning label similar to that shown inFIG. 3, where it is noted that the warning relates to “scatteredradiation” also, and the user of the laser is usually required to wearsafety googles and is typically a trained professional, all of theseaspects being very different from the acceptable conditions of use of adomestically available laser power transmission system for chargingmobile electronic devices.

The prior art typically uses anti-reflective coatings on surfaces toprevent such reflections, in combination with elaborate beam blockingstructures to block such reflections, should they nevertheless occur.However, the AR-coating solution used in the prior art is prone tofailure from dust or spilled liquid deposited on its surface, or fromcoating wear and tear, such as from improper cleaning. Additionally, thebeam block solution typically limits the field of view of the systemseverely, and is bulky compared to the dimensions of modern portableelectronic devices.

The prior art therefore lacks a reliable and “small footprint” mechanismto prevent scattering and reflections from the power beam in unwanteddirections. Such scattering and reflections may be caused either by atransparent surface inadvertently placed between the transmitter and thereceiver, and the optical characteristics of that transparent surfacemay arise from a vast number of different transparent materials, or fromliquid spills and fingerprints which may be deposited on the externalsurfaces of the system, typically on the front surface of the receiver.

A third problem with the solutions suggested in the prior art, is thatsuch safety systems generally require a mechanism to guarantee goodalignment of the power beam system and the safety system such that bothsystems are boresighted on the same axis until the power beam divergesenough or is attenuated enough (or a combination of these factors andany other factors) so that it no longer exceeds safety limits. This isextremely difficult to achieve with a collimated class IV or IIIb laserbeam, which typically expands very little with distance and thus exceedsthe safety limit for a very long distance.

One prior art principle of operation used to build such a safety systemis the optical detection of transparent surfaces that may be positionedin the beam's path. However transparent surfaces that may enter the beampath may be made from a vast number of different transparent materials,may be antireflection AR coated or may be placed in an angle close toBrewster's angle so they are almost invisible to an optical systemunless they absorb the beam. However, since light absorption levels foreach material are different, and may even be negligible, and sincebuilding an optical system that relies on optical absorption will behighly material specific, and since the number of available materials isextremely large, such a system is likely to be complex, large andexpensive, and unless properly designed, is likely to be unreliable,especially when considering that it is meant to be a critical safetysystem. Relying on the reflections to provide detectable attenuation ofthe beam is also problematic, as the surfaces may be coated by ananti-reflective coating or positioned in a near Brewster angle to thebeam, such that the reflection may be minimal for that particularposition of the surface.

There therefore exists a need for a laser power transmission system withbuilt-in safety features, which overcomes at least some of thedisadvantages of prior art systems and methods.

The disclosures of each of the publications mentioned in this sectionand in other sections of the specification, are hereby incorporated byreference, each in its entirety.

SUMMARY

Since many of the reflective materials in domestic use are plastic, thepresent disclosure attempts to provide a system in which the light beamused to power the remote device is absorbed by the vast majority oftransparent organic materials, such that it would be simple to detectwhen a plastic or other organic material is inserted into the beam. Thisshould be applicable even for a plastic material essentially transparentat some wavelengths. Achievement of this aim would result in a laserpower transmitting system that would provide good protection frominadvertent reflections from supposedly transparent plastic objectsinserted into the beam.

It is not feasible to measure the absorption/transmission spectrum ofall transparent materials to determine their optical properties, asthere are too many such materials, many of which do not have readilyavailable absorption spectra in the literature, which could be used forevaluation. A more theoretical, systematic approach is therefore needed.

Opaque or even partially opaque materials can be easily detected whenplaced in the beam, by measuring the beam's attenuation. However somematerials are transparent or nearly transparent and it is suchtransparent materials that are significantly harder to detect. There aretwo major groups of solid transparent materials, organic and inorganicmaterials. The number of inorganic transparent solid materials availableto the general public is fairly limited, consisting mostly of glasses, afew semiconductor materials in common use, quartz, and some naturallyoccurring minerals such as diamonds, ruby and calcite. It is thereforepossible to build a detection system for reflections from inorganictransparent materials, covering all likely scenarios.

On the other hand, the availability of different organic, transparentmaterials to the general public is enormous, and new transparentmaterials are being added to the list all the time. This is asignificant problem as characterizing this group optically is thusvirtually impossible.

Polymers are a significant group of transparent organic materials, andthey will be used as a sample group to assist in explaining the way inwhich the current invention is intended to operate. Polymers typicallyconsist of long chains of monomers, with the backbone of such polymersbeing typically made of either carbon or silicon. FIGS. 4 to 9 show thechemical structure of some commonly used transparent polymers. FIG. 4shows a Poly-methyl methacrylate (PMMA) chain; FIG. 5 shows thestructure of a polycarbonate; FIG. 6 shows the polystyrene structure;FIG. 7 shows nylon 6,6; FIG. 8 shows a polypropylene chain; and FIG. 9shows the polyethylene chain structure.

As is observed, the chemical structure of the sample polymers shown isvery different, and the absorption spectra of these polymers depend onmany factors including the density of the material, trace amounts ofreagent, and the chain length. Yet it is observed that all the abovetransparent polymers have some chemical bonds in common, especially C—Cand C—H bonds. This is especially true for commercially availablepolymers, which are almost entirely based on organic materials, whichwould be detected by the systems of the present disclosure, orsemi-organic silicon based polymers such as silicones, polysilanes,polygermanes and polystannanes, or polyphospahazenes, which would alsobe detected by systems of the present disclosure.

Apart from that, the number of transparent materials available to thegeneral public which is not based on carbon chemistry is fairly limited,consisting mostly of various glasses, most of which have readilyavailable data on their transmission spectra.

If a system were designed so that the laser excites either a vibrationalC—H, or also possibly a C—C bond in polymers, then it would be easy todetect when one such polymer were positioned within the beam, bymonitoring the power drop caused by the polymer. This assumes that theabsorption of the C—H or C—C bond is always present and is alwayswavelength aligned to the laser wavelength. Rotational peaks could alsobe used for this purpose, but they may be unreliable in polymers, suchthat the vibrational C—H (or C—C) absorptions are better suited for thispurpose.

Reference is now made to FIG. 10, which shows a chart of typicalabsorption regions of different polymer bonds. It is observed that theC—H stretch vibration around 2900-3200 cm⁻¹, appears in almost all ofthe polymers shown. This could therefore be used as the absorptionmechanism trigger for a safety system, using the change in transmittedpower resulting from the absorption bands. However, there are twoproblems with these absorption bands, which make them less useful forthis purpose:

-   -   (i) The C—H vibrational absorption lines are typically very        sharp, and their exact frequency varies much from one polymer to        another, so a laser may excite one polymer, but not another.        Thus, unless the laser is tuned exactly to the specific C—H        vibration line of that polymer, it would not be absorbed.    -   (ii) Such C—H vibration peaks are generally medium absorption        peaks, meaning that the attenuation of a beam due to a material        section a few mm thick would be 20-50% (i.e. it allows detection        of even trace amounts of material in a small container), and        while medium (20-70% attenuation per cm material) and strong        (>70% attenuation per cm) absorption peaks are generally much        easier to detect, they cannot be used to construct a robust        system.

In a commercial system, designed for the consumer environment,fingerprints are a common problem. In normal operation the system shouldnot fail simply when a fingerprint is deposited upon it; instead thesystem should shut down transmission when there is a risk of exceedingsafety limits. To do this, the system should detect blocking of the beambut should not cease transmission due to any fingerprint deposited onthe receiver. If a strong or medium absorption peak is used, then shoulda fingerprint or some other contamination be deposited on the externaloptical surface of the receiver or transmitter, it would absorb the beamsignificantly, causing power transmission to fail. This arises sincefingerprints also contain organic compounds that would absorb the beam,resulting in uncontrolled system failure. In order to allow the systemto operate in an environment where organic materials such asfingerprints may be deposited on the surface of its typically externaloptical components, it would be necessary to build a system where thelaser beam successfully traverses the finger print, while the safetysystem detects dangerous transparent items that may be inserted into thebeam. If, on the other hand, a safety system were to utilize a weakabsorption band instead of a medium or strong one, then the systemshould continue to operate with the fingerprint, and shutoff may be donebased on an electronic decision and not in an uncontrolled manner.

Turning to the C—C absorption band, stretching from 800 cm⁻¹ to 1300cm⁻¹, this is such a wide band that a narrowband laser is almost certainto miss a narrowband absorption peak in this region, since while thepeak may be positioned in the 800 cm⁻¹ to 1300 cm⁻¹ range, its typicalwidth is very small, and may be easily missed by a narrowband laser.Additionally, as will be seen in FIG. 11 hereinbelow, this band vanishesfor some polymers, where no absorption peak is visible between 800 and1300 cm⁻¹ and some polymers may exist where C—C bonds are not present,and are replaced by aromatic carbon-carbon bonds or by C═C bonds andC—O—C bonds

An additional problem arises from the absorption strength of the C-Cline. In symmetrical compounds such as polyethylene, it may be nearlyimpossible to detect, while in other compounds it may be so strong thateven a weak fingerprint on the surface of the receiver will make thesystem inoperable, as a significant portion of the power may be absorbedby the fingerprint, making the device unusable. To enable operation of asystem in which fingerprints may be deposited on its optical surfaces, aweak, but not too weak absorption line is required that will not changemuch between different polymers and which would be found in most organicpolymers, and a laser tuned to that peak should be used, in conjunctionwith a system that operates around that peak. As can be seen from FIG.10, there is no such peak in the commonly used polymers and in theabsorption bands shown.

There is thus provided in accordance with an exemplary implementation ofthe systems described in this disclosure, a system for optical wirelesspower transmission to a power receiving apparatus comprising:

-   -   (a) an optical resonator having end reflectors and adapted to        emit an optical beam,    -   (b) a gain medium positioned inside the optical resonator and        having a first bandgap energy, the gain medium being thermally        attached to a cooling system and configured to amplify light        passing through it,    -   (c) a driver supplying power to the gain medium, and controlling        the small signal gain of the gain medium,    -   (d) a beam steering apparatus configured to direct the optical        beam in at least one of a plurality of directions,    -   (e) an optical-to-electrical power converter configured to        convert the optical beam into electrical power having a voltage,        the optical-to-electrical power converter having a second        bandgap energy,    -   (f) an electrical voltage converter, adapted to convert the        voltage of the electrical power generated by the        optical-to-electrical power converter into a different voltage,        the electrical power converter comprising an inductor, an energy        storage device and a switch,    -   (g) at least one surface associated with the        optical-to-electrical power converter and optically disposed        between the gain medium and the optical-to-electrical power        converter,    -   (h) a detector configured to provide a signal indicative of the        optical beam impinging on the optical-to-electrical power        converter, and    -   (i) a controller adapted to control at least one of the status        of the beam steering apparatus and the driver, the controller        receiving a control input signal from at least the detector,        -   wherein:    -   (j) the at least one surface has properties such that it        reflects a small part of light incident on it, either (i)        diffusively, or (ii) such that the reflected light has a virtual        focus positioned remotely from the optical resonator relative to        the surface, or (iii) such that the reflected light has a real        focus positioned at least 1 cm. in the direction of the optical        resonator relative to the surface,    -   (k) the controller is configured to respond to the control input        signal received from the detector by at least one of (i) causing        the driver to change the small signal gain of the gain        medium, (ii) changing the radiance of the optical beam, (iii)        changing the power supplied by the driver, (iv) changing the        scan speed of the beam steering apparatus, (v) changing the scan        position of the beam steering apparatus, and (vi) recording a        scan position defining the position of the optical-to-electrical        power converter,    -   (l) the gain medium is a semiconductor device or a solid host        doped with Nd ions, and includes a filter attenuating radiation        for at least one frequency having a wave number in the range        8,300 cm⁻¹ to 12,500 cm⁻¹,    -   (m) the second bandgap energy is smaller than the first bandgap        energy,    -   (n) the first bandgap energy is between 0.8 eV and 1.1 eV,    -   (o) the switch has a closed serial resistance smaller than R,        given by the equation:

$R \leq \frac{E_{gain}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}$

where R is measured in Ohms, E_(gain) is the first bandgap energymeasured in Joules, and

P_(laser driver) is the power supplied by the laser driver to the gainmedium, measured in Watts, and

-   -   (p) the optical beam has a radiance of at least 8        kW/m²/Steradian, and a frequency between the first overtone of        the C—H absorption situated at approximately 6940 cm⁻¹ and the        second overtone of the C—H absorption situated at approximately        8130 cm⁻¹.

In any such a system, the different voltage may be a higher voltage thanthe voltage generated by the optical-to-electrical converter.Furthermore, the status of the beam steering apparatus may be either orboth of the aiming direction and the scan speed of the beam steeringapparatus.

Furthermore, in any of the above-described systems the optical beam mayhave a radiance of at least 800 kW/m²/Steradian.

Another example implementation can involve any of the above describedsystems in which each one of the end reflectors of the resonator areeither: (i) dielectric mirrors, (ii) Bragg mirrors, (iii) Fresnelreflectors or (iv) mirrors composed of alternating layers of dielectricor semiconductor material having different refractive indexes.Additionally, the gain medium can be either a transparent solid hostmaterial doped with Nd ions or a semiconductor. In such a case, thesystem may further comprise a filter for extracting radiation having awave-number greater than 8300 cm⁻¹. In the event that the gain medium isa semiconductor, it may advantageously be a quantum dot gain medium.

In further exemplary implementations of the above described systems, thecooling system may be at least one of a heatsink, a Peltier diode, and aliquid cooled plate. It may also be equipped with a fan. Additionally,the gain medium may be attached to the cooling system using a layer ofsolder having less than 200° Kelvin/Watt thermal resistance. In anyevent, the cooling system may be such that the thermal resistancebetween the gain medium and the surrounding air is less than 200°Kelvin/Watt.

In alternative implementations of any of the above-described systems,the optical-to-electrical power converter may be a photovoltaic cell. Insuch a case, the photovoltaic cell may be a III-V device. In any event,the serial resistance of the optical-to-electrical power converter maybe less than 1 Ohm.

According to further implementations of the above described systems, theinductor should have a serial resistance measured in Ohms of less thanthe square of the first bandgap energy measured in Joules divided by2*10⁻⁴⁰ times the driver power measured in Watts.

In other implementations, the energy storage device may be either acapacitor or a rechargeable battery.

Additionally, any of the above described systems may further comprise aretro reflector. Also, the gain medium may be pumped electrically oroptically by the driver. Furthermore, the second bandgap energy may bemore than 50% of the first bandgap energy.

Yet other implementations perform a method for transmitting power from atransmitter to a receiver, comprising:

-   -   (a) converting a first electrical power to an electromagnetic        wave having a frequency between the first overtone of the C—H        absorption situated at approximately 6940 cm⁻¹ and the second        overtone of the C—H absorption situated at approximately 8130        cm⁻¹, the electromagnetic wave having a radiance of at least 8        kW/m²/Steradian, the converting being performed by using an        optical resonator having end reflectors and a gain medium        connected to a laser driver receiving the first electrical        power, the gain medium having a first bandgap energy between 0.8        eV and 1.1 eV, being positioned inside the optical resonator,        being thermally attached to a cooling system, and configured to        amplify the electromagnetic wave passing through it,    -   (b) directing the electromagnetic wave into at least one of a        plurality of directions using a beam steering apparatus        controlled by a controlling unit,    -   (c) detecting the impingement of the beam on a target having an        associated partially transparent surface, such that an        indication relating to the impingement may be utilized by the        controlling unit to perform at least one of (i) causing a change        in the small signal gain of the gain medium, (ii) causing a        change in the radiance of the electromagnetic beam, (iii)        causing a change in the first electrical power, (iv) changing        the scan speed of the beam steering apparatus, (v) changing the        scan position of the beam steering apparatus, and (vi) recording        a scan position defining the position of the target,    -   (d) converting the electromagnetic wave into a second electrical        power having a voltage, by using an optical-to-electrical power        converter having a second bandgap energy smaller than the first        bandgap energy,    -   (e) converting the voltage into a different voltage using an        electrical voltage converter, comprising an inductor, an energy        storage device and a switch having a closed serial resistance        smaller than R, given by the equation:

${R \leq \frac{E_{gain}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}},$

where R is measured in Ohms, E_(gain) is the first bandgap energymeasured in Joules, and P_(laser) _(_) _(driver) is the first electricalpower, measured in Watts,

wherein:

-   -   (f) the surface is designed such that it reflects a small part        of the electromagnetic wave incident on it either (i)        diffusively, or (ii) such that the reflected light has a virtual        focus positioned remotely from the optical resonator relative to        the surface, or (iii) such that the reflected light has a real        focus positioned at least 1 cm. in the direction of the optical        resonator relative to the surface, and    -   (g) the gain medium is either a semiconductor device, or a solid        host doped with Nd ions that includes a filter attenuating        radiation for at least one frequency having a wave number in the        range 8,300 cm⁻¹ to 12,500 cm⁻¹,

In such a method, the switch may be switched at a frequency determinedby the equations:

$f < {\frac{1}{1.28*10^{- 40}*L}*E_{gain}^{2}*\frac{1 - \frac{E_{gain}}{5*10^{- 19}*V_{output}}}{P_{{laser}\_ {driver}}}}$$f > {\frac{1}{3*10^{- 38}*L}*E_{gain}^{2}*\frac{\left( {1 - \frac{E_{gain}}{4*10^{- 20}*V_{output}}} \right)}{P_{{laser}\_ {driver}}}}$

where f is the switching frequency measured in Hz., E_(gain) is thebandgap of the gain medium, measured in Joules, V_(output) is the outputvoltage from the voltage converter, measured in Volts, andP_(laser driver) is the power pumped by the laser driver onto the gainmedium, measured in Watts.

Additionally, the detection of impingement of the beam on the target maybe done using either detection in the transmitter of retro reflectedillumination from the target, or detection of illumination of the targetusing a receiver sensor.

Furthermore, in any of the above described methods, the second bandgapenergy may be more than 50% of the first bandgap energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 shows the energy density of various battery chemistries;

FIG. 2 shows the maximal permissible exposure value for lasers forvarious exposure times, according to the US Code of Federal Regulations,title 21, volume 8, (21 CFR §8), revised on April 2014, Chapter I,Subchapter J part 1040;

FIG. 3 shows an example of a warning sign for a class IV laser product;

FIGS. 4-9 show examples of the chemical composition of various commonlyused transparent polymers;

FIG. 4 shows a Poly-methyl methacrylate (PMMA) chain;

FIG. 5 shows the structure of a polycarbonate;

FIG. 6 shows the polystyrene structure;

FIG. 7 shows the structure of nylon 6,6;

FIG. 8 shows a polypropylene chain structure;

FIG. 9 shows the polyethylene chain structure;

FIG. 10 shows the IR absorption bands for common organic chemical bonds;

FIG. 11 shows the IR absorption spectrum of polyethylene;

FIG. 12 shows the overtone absorption bands for some common organicchemical bonds;

FIGS. 13A and 13B show different electronic configurations forconverting the output voltage of a photovoltaic cell to a differentvoltage;

FIG. 14 shows the power reflected per square meter by a mirror, when abeam of radiance 8 kW/m²/steradian is focused upon it, as a function ofnumerical aperture;

FIGS. 15A, 15B and 15C show schematic drawings of exemplary apparatusaccording to the present disclosure, for avoiding unsafe reflectionsfrom the front surface of a receiver being illuminated by a transmitterof the present disclosure;

FIG. 16 shows a schematic diagram showing a more detailed description ofthe complete optical wireless power supply system of the presentdisclosure;

FIG. 17 is a graph showing the change in power transmission of thesystem of FIG. 16, as a function of the angle of tilt of the beamsteering mirror; and

FIG. 18 shows a schematic representation of a cooling system for thegain medium of the system of FIG. 16.

DETAILED DESCRIPTION

In view of the above described considerations, one exemplaryimplementation of the optical wireless power supply systems of thepresent disclosure could be a system tuned to work in between the firstovertone of the C—H absorption at 6940 cm⁻¹ and the second overtone ofthe C—H absorption at 8130 cm⁻¹. Such overtone bands are less knownbands, containing much less chemical information, and arise fromessentially forbidden quantum mechanical transitions, and are onlyallowed due to complex mechanisms. Consequently, they provide wide, weakabsorption bands, exactly as preferred for this application, but havefound significantly less use in analytical chemistry. The broad natureof the bands allows for detecting various different polymercompositions, while the weak absorption allows the system to continueoperation even in the vicinity of organic dirt and fingerprints. Thismakes these lines significantly less useful for typical uses ofabsorption measurements, but ideal for the present task. Anotheradvantage of these lines is that there are no commonplace absorptionlines directly positioned at the same frequencies, so that changingchemical composition of the materials will not alter the measurementresults strongly. Many such overtone bands are illustrated in the chartof FIG. 12.

Electro-optical components that operate in that band are scarce and hardto source, probably since both diode lasers and diode-pumped, solidstate (DPSS) lasers are significantly less efficient at thosefrequencies, and only lower power lasers are currently commerciallyavailable. Since lasers at the preferred frequencies, with the desiredparameters, are, not currently available, a laser suitable for this usehas to be designed from the ground up. The resonator and gain mediumhave to be designed. A laser with the selected frequency and a radiancevalue sufficient to facilitate a roughly collimated or nearly collimatedbeam must be constructed. To achieve good collimation of the beam, aradiance of at least 8 kW/m²/Steradian is needed, and even 800kW/m²/Steradian may be needed for higher power systems for efficientpower transmission. For small systems working at long distances, muchhigher radiance (up to 10GW/m²/Steradian) may be designed in the future,according to similar principles. Receivers for use with radiance of lessthan that level need to be too large, which would make the systemcumbersome.

Different mirror setups for the resonator have been used, specificallygood quality metallic mirrors made of Gold, Silver or Aluminium. Theseare found to reduce the lasing efficiency significantly. Much betterresults are achieved with dielectric material mirrors. Alternatively,Fresnel mirrors have one advantage in that they are low cost. Othermirrors that may be used are Bragg mirrors (which may be dielectric).The mirrors need to be positioned so as to form a stable, or a nearlystable resonator, or a resonator where photons are confined in space bya barrier inside the laser (such as in a fiber or diode laser) and again medium should be placed in the resonator between the mirrors in aposition allowing the gain medium to amplify the beam resonating insidethe resonator, such that it has a radiance of at least 8kW/m²/Steradian.

If the gain medium is capable of lasing at more than one wavelength, thedielectric mirrors can be selected to limit that wavelength to aspecific value. Alternatively, a filter can be used to fix the lasingfrequency.

Specifically, it is better if the mirrors have high reflectivity for atleast one wavelength between the first overtone of the C—H absorption at6940 cm⁻¹ and the second overtone of the C—H absorption at 8130 cm⁻¹.

Three different approaches may be used for the gain medium.

1. DPSS design:

In the DPSS design, the gain medium may be a Nd-doped YAG crystal,though YVO₄ crystals, GGG crystals and Glasses are also options for aclear host. Neodymium is most suitable for operation between the firstovertone of the C—H band and the second overtone of the C—H band sinceNd has a transition near ˜7450 cm⁻¹. The Nd ions need to be excited byabsorbing radiation, typically from 808 nm laser diodes, although otherwavelengths may be used. A Nd-based gain medium tends to laser at a muchhigher frequency unless a filter blocking the transition around 9400cm⁻¹ is added inside the resonator, or unless the unwanted radiationfrom the resonator is otherwise extracted. When such a filter is added,lasing commences at 7440-7480 cm⁻¹. Such filter action can be achievedusing a prism or a grating, instead of a filter or by proper chromaticdesign of the laser resonator.

2. Semiconductor laser

As an alternative, a semiconductor-based design may be proposed. It ispossible to tune the wavelength of semiconductor lasers by altering thelasing bandgap of the semiconductor used. Semiconductors, especiallythose of the III-V type and more especially, though not exclusively,quantum dot types, having bandgaps of the order of 1 eV, emit light atthe desired frequencies between 6900 cm⁻¹ and 8200 cm⁻¹. Specificallybandgaps between 0.8 eV and 1.1 eV yield good results and are absorbed,at least partially, by essentially all commonly used polymers.

3. Various alternative designs may also be used in the systems describedin this disclosure, such as Nd doped fiber lasers that may include Braggmirrors and/or fiber loop mirrors. Alternatively Raman shifted fiberlasers may also be used.

During operation the gain medium heats up, and should be cooled toprevent wavelength shift and efficiency degradation. If the gain mediumis properly cooled, then it is possible to increase the pump power orcurrent until a beam having radiance of at least 8 kW/m²/Steradian isemitted, having a frequency between 6900 cm⁻¹ and 8200 cm⁻¹. Such a beamcan be nearly collimated and will be attenuated by most organicmaterials, including polymers, allowing detection. However, it will notbe strongly absorbed by contaminations such as fingerprints.

The laser gain medium is typically configured to work at a temperaturebelow 150 degrees centigrade. If its temperature exceeds a level,typically around 250 centigrade, a number of problems arise.

Firstly, the efficiency of light emission may drop significantly, due topopulation of lower level excited states, especially in 3- and 4-levellasers, and also due to thermal recombination of charge carriers insemiconductors.

Secondly, the soldering of the gain medium, if such a thermal attachmentmethod is used, may be damaged.

Thirdly, thermal aberrations may occur which may cause beam degradation

Fourthly, the thermal expansion of the laser gain medium may bedifferent from that of its surroundings, which may cause mechanicalstress or even warping and fracture of the gain medium.

For those reasons, inter alia, the gain medium has to be thermallyattached to a cooling system. Typically the gain medium emits between0.1 and 100 W of heat from a surface that is between 1 mm² and 40 mm².In order for the temperature of the gain medium to be maintained at lessthan 150 degrees, the cooling system of the gain medium needs to have athermal resistance of less than 200 Kelvin per Watt, and for systemstransmitting higher powers, typically arising from more than 10 W ofelectrical power input, the thermal resistance should be significantlylower, and in many cases thermal resistance lower than 0.05 Kelvin/Wattis needed.

The surface of the cooling system is attached to the gain medium,typically using a third material such as solder or adhesive, which musthave an expansion coefficient that is compatible to both the expansioncoefficient of the gain medium itself and to the expansion coefficientof the front surface of the cooling system.

Typically such cooling systems may be either a passive heat sink, a heatsink with a fan, a Peltier element connected to a heat sink with orwithout a fan, or a liquid cooled cooling system. Alternatively, use maybe made of a stand-alone liquid circulating cooling system with activecirculation based on a circulating pump, or with passive circulation,based on heat pipes.

If the cooling system consists of a heat sink with a fan, its thermalresistance should be less than 0.1° Kelvin per Watt

If the cooling system is a passive heat sink, its thermal resistanceshould be less than 0.3° Kelvin per Watt

If the cooling system is a Peltier element, it needs to generate atleast 5 degrees temperature difference ΔT.

If the cooling system is an active liquid cooled cooling system, itshould be able to cover the entire span of thermal resistances mentionedhere.

A passive heat sink is preferred in systems designed for low cost andquiet operation, while a liquid cooled system is preferred for highpower systems. A heat sink with a fan or a fluid pump is used forsystems typically having more than 1 W electrical output and atransmitter having a small volume, such as less than approximately 1liter.

The gain medium is typically driven by a driver, supplying it withpower, which may be provided as electrical power as in the case of somesemiconductor gain media, or optical as in the case of othersemiconductor gain media or DPSS systems, or chemical or other forms ofenergy. The amount of power supplied by the driver determines the smallsignal gain achieved, which determines the working conditions andemission of the laser, while the saturated gain of the gain medium isgenerally a function of the material selected for the gain medium,though not always in a simple linear fashion, and ultimately, theradiance emitted from the laser. Such a laser driver might have two ormore operational states, one used for power transmission, and the othersused for other functions of the system, such as target seeking, setup,and information transmission. It is important that the laser driverproduces stable emission (with regards to power and beam parameters) inboth working conditions, although stable operation during powertransmission is more important.

To convert the optical beam into electricity again, so that useful poweris delivered, an optical-to-electric power converter, typically aphotovoltaic cell, should be used. As with the lasers, suitablephotovoltaic cells tailored to the frequency of the beam used, are notcommonly available as off-the-shelf components, and a custom cell isrequired. The bandgap of the photovoltaic semiconductor should beslightly smaller than the bandgap of the gain medium used, so that thebeam frequency is absorbed efficiently by the semiconductor. If not, theconversion efficiency will be very poor. On the other hand, if thebandgap used is too small, then a poor efficiency system is achieved.Also the conductors on the photovoltaic cell need to be tailored to theradiance of the beam used—the higher the radiance, the thicker theconductors needed.

Since the bandgap of the laser gain medium should be in the range0.8-1.1 eV, and the bandgap of the photovoltaic cell used must be lower,and since a single junction photovoltaic cell typically produces avoltage that is about 60-80% of the bandgap energy divided by theelectron charge, a single junction cell tailored to the laser frequencyyields a very low voltage, typically 0.3-0.8V, and typically a highcurrent, assuming an output power of a few watts, as required by apractical system. The conductors on the semiconductor need to be thickenough to carry the generated current without significant (e.g. >5%)losses. Typically the series resistance of the conductors needs to bebelow 1 Ohm, or even better, below 0.1 Ohm, and the heat generatedshould be efficiently extracted from the photovoltaic cell as itsefficiency generally decreases with temperature.

This combination of low voltage combined with high power cannot beeasily converted to the higher voltages required to charge portabledevices, typically 3.3 or 5V. Furthermore, some systems, such ascommunication systems, may require voltages such as −48V, 12V or 3.8V.The system needs to supply a stable voltage, and at higher levels thanthe output voltage expected from the photovoltaic cells. A typicalmethod to increase the voltage of photovoltaic cells is to connect themin series, such as is described in U.S. Pat. No. 3,370,986 to M.F.Amsterdam et al, for “Photovoltaic Series Array comprising P/N and N/PCells”, which shows a typical configuration for yielding a highervoltage, while utilizing almost the same amounts of semiconductor and noadditional components, and is therefore the typically chosen solution.

However this solution is not suitable for systems such as thosedescribed in the present application, in which a laser having a radianceas high as 8 kW/m²/Steradian is used, especially since such a lasertypically does not have a uniform shaped beam. Furthermore, its beamshape may be variable in time and the pointing accuracy may be less thanoptimally desired. In such a situation it is virtually impossible todesign a compact and efficient system that will illuminate all the cellsuniformly. If the photovoltaic cells connected in series are notuniformly illuminated, they do not produce the same current. In such acase the voltage will indeed be increased to the desired level but thecurrent would drop to the current generated by the cell producing theleast current, usually the cell least illuminated. In such a situationefficiency will be very poor. There is thus a need for an improvedalternative method to increase the voltage.

One method of increasing the voltage of a single cell may be by chargingcapacitors in parallel, and then discharging them in series. This methodyields good results for low currents, but when current is increasedbeyond a certain level, the switching time becomes a dominant factor,influencing efficiency, which degrades with increasing switching time.

If the energy is converted to AC using a fast, low resistance, switchingmechanism, that AC current can be amplified using coupled inductancesand then converted to AC again. The increased voltage AC can beconverted to DC using a diode bridge and an energy storage device, suchas a capacitor or a battery. Such systems have advantages when thevoltage needs to be increased beyond twenty times that of the photocellvoltage. Another advantage of such a system is that the switching can bedone from the transmitter using the laser, thus saving receiver cost andcomplexity. Such systems have disadvantages when the voltage needs to beincreased by a factor of less than 10 or when size and volumelimitations are critical to the application.

Reference is now made to FIG. 13A, which shows a method of voltageconversion that is efficient and simple. In the configuration of FIG.13A, a single inductor may be used with a low resistance switchingmechanism and an energy storage device to increase the voltage of thephotovoltaic cell. In FIG. 13A, the square on the left is thephotovoltaic cell, the switch S, is a low resistance switch, such as aMOSFET, JFET, BJT, IGBT or pHEMT, the inductance L is connected to theoutput of the photovoltaic cell and the capacitor C acts as an energystorage device.

The following description assumes for simplicity the use of componentswith zero resistance. Taking resistance losses into account complicatesthe calculations, and is explained in a later section of thisdisclosure. The switching mechanism cycles the inductor between twoprimary operating phases: charging phase and discharging phase. In thecharging phase the inductor is connected in parallel with thephotovoltaic cell, by the closing of switch S. During this phase theinductor is being charged with the energy converted by the photovoltaiccell. The inductor energy increase is given by:

ΔE _(L) _(_) _(CH) =Vpv*I _(L) *T _(CH),

Where:

Vpv is the output voltage of the photovoltaic cell,

I_(L) is the average inductor current, and

T_(CH) is the duration of the charging phase.

In the discharging phase, the inductor is connected between thephotovoltaic cell and the load by the opening of switch S. During thisphase, the energy delivered from the inductor to the output energystorage device is given by the inductor energy decrease:

ΔE _(C) =Vo*I _(L) *T _(DIS), where:

-   -   Vo is the voltage of the energy storage device, which is        typically very close to the desired output voltage of the        device, and can therefore be approximated as the output voltage        of the system,    -   I_(L) is the average inductor current, and    -   T_(DIS) is the duration of the discharging phase.

The energy delivered from the photovoltaic cell to the inductor duringthat phase is given by:

ΔE _(L) _(_) _(DIS) =Vpv*I _(L) *T _(DIS).

The change in the inductor energy during that phase is the differencebetween the incoming and outgoing energy:

ΔE _(L) _(_) _(DIS) =Vpv*I _(L) *T _(DIS) −Vo*I _(L) *T _(DIS).

In steady state operation, the energy of the inductor at the end of thecycle returns to the same value it was at the beginning of the cycleyielding:

ΔE _(L) _(_) _(CH) =−ΔE _(L) _(_) _(DIS),

Which, after substitution, yields:

Vo=Vpv*(1+T _(CH) /T _(DIS)).

The voltage at the energy storage device is thus defined by thephotovoltaic cell voltage and the ratio of the charging and dischargingphase durations.

In the present system, however, the parasitic characteristics and otheraspects of the components might have a significant impact on conversionoperation and efficiency and care should be taken into account inselecting and using the right components, in order to allow the systemto operate efficiently. These elements are now considered, one by one:

Inductor

-   -   1. The inductance of the inductor defines the rate of change of        the inductor current due to applied voltage, which is given by        dI/dt=V/L, where dI/dt is the rate of current change, V is the        voltage applied across the inductor and L is the inductance. In        the context of the current system, V is determined by the gain        medium in the transmitter. Selection of a different gain medium        causes change in the photon energy, which mandates consequent        changes in the photovoltaic bandgap, and hence a change in the        photovoltaic voltage. This then calls for selection of a        different inductor and/or switching frequency. The switching        rate must be fast enough to allow the inductor current to        respond to changes in the incoming power from the transmitter        through the optical-to-electrical power converter, and slow        enough to avoid high-magnitude current ripple which contributes        to power loss, input voltage ripple and output voltage ripple.        The optimal value of the inductor should yield ripple current        which is between 20% and 40% of the maximum expected input        current, but systems may be operable between 10% and 60%.        Rigorous analysis of the circuit parameters shows that in order        to achieve this objective, the value L, of the inductor measured        in Henries, must be within the limits:

$L < {\frac{1}{1.28*10^{- 40}*f}*E_{gain}^{2}*\frac{1 - \frac{E_{gain}}{5*10^{- 19}*V_{output}}}{P_{{laser}\_ {driver}}}}$$L > {\frac{1}{3*10^{- 38}*f}*E_{gain}^{2}*\frac{\left( {1 - \frac{E_{gain}}{4*10^{- 20}*V_{output}}} \right)}{P_{{laser}\_ {driver}}}}$

Where:

-   -   f is the switching frequency measured in Hz.,    -   E_(gain) is the bandgap of the gain medium, measured in Joules,    -   V_(output) is the output voltage from the voltage converter,        measured in Volts, and    -   P_(laser) _(_) _(driver) is the power pumped by the laser driver        into the gain medium, measured in Watts.

In order to successfully integrate the inductor into a mobile client,the inductance should typically be smaller than 10 mH, as inductors thatare suitable for the current required by mobile client charging andhaving volume limitations suitable for a portable application aretypically well below this value. Also inductors having inductances toosmall, such as 10 nH, will require such a high switching frequency thatit will severely limit the availability of other components in thesystem such as the switch, and the switching loss caused by such a highfrequency may be higher than the amount of power delivered by thephotovoltaic cell.

-   -   2. The serial resistance of the inductor, R_(parasitic), should        be as low as possible to minimize the conduction power loss:        Typically, a value which yields less than 10% efficiency drop is        chosen: the serial resistance of the inductor, measured in Ohms        should be less than

$R_{parasitic} \leq {\frac{1}{2*10^{- 40}}*\frac{E_{gain}^{2}}{P_{{laser}\_ {driver}}}}$

Where:

-   -   E_(gain) is the bandgap of the gain medium, measured in Joules,        P_(laser) _(_) _(driver) is the power pumped by the laser driver        onto the gain medium, measured in Watts.    -   3. In a typical system the inductor serial resistance would be        less than 10Ω. The saturation current of the inductor is usually        chosen to be higher than the expected inductor peak current,        given by:

I _(SAT) >I _(PEAK) =Im+Vpv*(1−Vpv/Vo)/(2*L*f).

For extracting more than 10 mW of power from a single junctionphotovoltaic cell, the saturation current must be higher than 10 mW/0.8v=12.5 mA.

-   -   4. For reliable operation the inductor shall be rated at a        higher current than the expected maximum input current. For        extracting more than 10 mW of power from a single junction        photovoltaic cell, the inductor rated current must be higher        than 10 mW/0.8 v=12.5 mA.

Switching Mechanism

-   -   1. The switching mechanism is usually made of two or more        devices. The first device, a main switch, when conducting, sets        the inductor into the charging phase. The second device can be        either a diode (as in FIG. 13A) or a switch whose function is to        connect the inductor to the load or output energy storage        device, during the discharging phase, and to disconnect it from        the load during the charging phase.    -   2. The switching mechanism should have low switch node        capacitance to minimize switching losses:

P _(SW2)=0.5*Csw*Vo ² *f.

For extracting more than 50% of the laser power, the switch nodecapacitance should be less than

${Csw} \leq {\frac{P_{{laser}\mspace{14mu} {driver}}}{V_{o}^{2}*f}.}$

-   -   3. In a typical system switch node capacitance would be less        than 100 nF and more than 10 pF.    -   4. The serial resistance of the main switch in the switch node,        that switch being either that connecting the inductor to the        ground or that connecting the optical-to-electrical power        converter to the inductor, should be less than

$R \leq \frac{E_{gain}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}$

In a typical system the switch serial resistance would be less than 10Ω.

Energy Storage Device

-   -   1. The energy storage device can be either a capacitor or a        battery or both.    -   2. The energy storage device is required to maintain the output        voltage during the charging phase, when the inductor is        disconnected from the output. The capacitance of the storage        device is chosen based of the switching frequency, laser power        and the desired output ripple voltage:

C _(OUT) >P _(LASER DRIVER)/(f*Vo*ΔVo)

Where ΔVo is the desired output ripple voltage.

-   -   3. The energy storage device can also supply power to the load        during temporary interruption of the optical path. For        uninterrupted power supply, the energy storage device should be        able to store at least the amount energy equal to minimal        operational output power (P_(OUT) _(_) _(MIN)) multiplied by the        interruption time interval (T_(INT)):

E _(OUT) _(_) _(MIN) ≧P _(OUT) _(_) _(MIN) *T _(INT).

If a capacitor is used as the energy storage device, the capacitanceshould be larger than:

C _(OUT)≧2*E _(OUT) /V _(OUT) ².

For uninterrupted operation at minimal operational output power largerthan 10 mW and interruption time interval longer than 100 ms the storedenergy has to be larger than 1 mJ and the capacitance larger than 80 μF(assuming V_(OUT)=5V).

In some cases, the capacitor may serve as the energy storage device forthe client application. In such cases, the client application may bedesigned without any secondary energy storage device (the conventionallyused battery installed in the mobile device), and the energy storagedevice of the presently described systems would have to store enoughenergy to supply the power needs of the client device until the nextcharging event. In such cases, super capacitors having a capacitance atleast 0.5F, and even above 10F, may be used. In other cases, where thepower requirement of the client device is low, or when it has anindependent energy storage device such as the battery internallyinstalled in the device, or if the device does not need to operate whenno power is supplied, the capacitor used would typically be well beyond1F. If a rechargeable battery is used as the energy storage device,then, similar to the capacitor logic above, if the battery is used onlyas means of regulating the voltage, but not as the means for maintainingpower supply to the client device between charging events, then theenergy capacity of the battery may advantageously be up to 100 times theenergy supplied during 100 cycles of the switch (typically below 0.1Wh), this level being determined according to the volume budget and costeffectiveness of the battery. On the other hand, if the battery is alsoused to power the client device between charging events, its capacityshould be at least large enough to store the energy needed by the clientdevice between charging events—typically above 0.1 Wh in the case of acellular phone. Batteries also have a volume limitation depending on theproduct in which they are intended to be used. Thus, the battery of aproduct that has some volume V, if incorporated externally to thedevice, would typically be limited to up to times the volume of thedevice, i.e. 3V. As an example of this rule of thumb, a battery used topower a cellular phone of 100 cc. volume would typically be limited toless than 300 cc. in volume. Such a battery would typically have acapacity of below 300 Wh. because of the above mentioned limitation.

The circuit in FIG. 13A is not the only possible topology. FIG. 13Bshows a different design that can achieve similar performancecharacteristics. The components roles, constraints and expected valuesfor FIG. 13B are the same as those listed for the circuit in FIG. 13A.The primary difference is that the positive and negative terminals ofthe output voltage are reversed.

In some applications the energy storage device may be preferably locatedinside the device which is intended to use the power received. In otherapplications, specifically those applications where short term operationis anticipated, and which does not require a regulated voltage, theenergy storage device may even be eliminated.

Point of Regulation

The power output of the photovoltaic cell depends on the incomingoptical power and load applied to it. The optimal loading condition willyield the maximal output power from the photovoltaic cell, therefore,the control mechanism of the voltage converter must regulate the loadingpoint. The control mechanism can be either designed to maintain constantvoltage between the cell terminals, which is known to be maximum poweroperating point for most conditions, or it can track the maximum poweroperating point by measuring the cell output power and seeking theoptimal cell voltage under any operating condition. The first approachis simpler; the second is more power efficient.

The generated laser beam needs to be directed towards the receiver. Inorder to direct the beam towards the receiver, a beam steering apparatusshould be used. Some beam steering sub-systems that could be usedinclude a moving mirror, a moving lens, an electro-optical modulator, amagneto-optical modulator, a set of motors moving the whole transmittersystem in one or more directions, or any other suitable beam deflectiondevice.

The beam steering apparatus should be controlled by a controller, mostconveniently the same controller used to control the laser driver.

The beam steering apparatus is configured to direct the >8kW/m²/Steradian beam in any of a number of directions.

The damage threshold of the beam steering apparatus needs to be able towithstand the beam's radiance.

For example, if the beam is focused on a mirror using a focusingmechanism having a numerical aperture of 0.5, the mirror needs towithstand a power density of at least 6.7 kW/m² for a beam having 8kW/m²/steradian. If a beam having higher radiance is used the mirrorshould be chosen so that it would have a higher damage thresholdcorrespondingly.

FIG. 14 shows the power reflected per square meter by a mirror when abeam of 8 kW/m²/steradian is focused upon it as a function of numericalaperture.

If a higher radiance beam is used, then the power reflected by themirror increases correspondingly in a linear manner.

Since the beam may be far from uniform, “hotspots”, sometimes having 10×irradiance compared to the beam average, may be generated.

Hence, mirrors should have a damage threshold which is at least as largeand preferably at least 10× that shown in FIG. 14, scaled to the actualbeam irradiance and numerical aperture of the focusing mechanism on themirror.

There is typically an optical front surface in the receiver, positionednear the photovoltaic cell and between the photovoltaic cell and thetransmitter, through which the beam enters the receiver, and which isneeded in order to protect the typically delicate structure of thephotovoltaic cell, and in many cases, in order to match the exteriordesign of the device where the power receiver is integrated in. Thefront surface may have a coating protecting it from scratches, such asCorning Gorilla Glass®, or similar, or may be treated to make it betterwithstand scratches. It may be also be treated to reduce the levels ofcontaminants, such as fingerprints and dust which may settle on it, orto reduce their optical effect, or it may be coated with ananti-reflection coating to reduce the level of light reflected from it.The front surface of the photovoltaic cell may also be coated. In somecases the front surface would be part of the structure of thephotovoltaic cell itself or coated on the photovoltaic cell.

While in some situations, it may be possible to reduce the amount ofreflection from the surface to below the safety threshold, by choosing avery low reflection anti-reflective coating, should the coating becontaminated or covered by either a liquid spilled on it, or afingerprint, such anti-reflective coating would be ineffective inreducing the amount of reflection, and typically, 3-4% of the incidentlight will be reflected in an uncontrolled direction. If such areflection is reflected in a diverging manner, its power density wouldsoon drop to safe levels. However, should the reflection be focused, thepower density may increase to unsafe levels. For this reason, it isimportant that the ROC (radius of curvature) of such a surface, at anypoint on it, should not be less than a predetermined value. In general,the reflection from the surface is intended to be only a small part ofthe incident light, thereby reducing the danger of any significant beamreflections, regardless of what nature or of what form the surfacecurvature takes. The level of reflected light may be variable, sinceeven the ˜4% reflection from an untreated glass surface may beincreased, if a layer of extraneous contaminant material on the surfacegenerates increased reflectivity. However, that reflection is expectednot exceed 20%, and will generally be substantially less than the 4% ofuntreated glass, such as in the case of AR coated glass, wherereflectivities of 0.1% or even less are common. Therefore, the surfaceis described in this disclosure, and is thuswise claimed, as havingproperties which reflect a small part of the incident light, thisdescription being used to signify less than 20% of the incident light,and generally less than the 4% of untreated glass.

Reference is now made to FIGS. 15A to 15C, which illustrateschematically methods of avoiding the above-mentioned unsafereflections, even for the small part of the incident light which may bereflected from the surface. FIG. 15A shows a situation where the surfaceis a concave surface, FIG. 15B shows a situation where the surface is aconvex surface, and FIG. 15C shows the situation where the surface is adiffusive surface. In FIG. 15A, an incident beam 110, having at least 8kW/m²/Steradian radiance, is directed towards photovoltaic cell 112,passing through a front surface 111, which may be the front surface ofthe photovoltaic cell. The front surface 111 reflects some of beam 110creating a focused beam 113 with a focal point 114 some distance fromthe surface. In order to ensure that focal point 114 does not presentany danger to an eye or skin, or other objects, the Radius of Curvature(ROC) of the surface 111 must be such that the beam is focused with lownumerical aperture, as in FIG. 15A, or that it be defocused, as in FIG.15B, or that it be diffused, as in FIG. 15C. To achieve theselimitations, if the surface is concave looking from the transmittertowards the photovoltaic cell, as in FIG. 15A, its ROC must be largerthan 1 cm, and if higher power systems are used, typically above 0.5 Wof light, it should be greater than 5 cm. Alternatively, the surface ROCcan be negative, as in FIG. 15B, but the ROC cannot be in the range 0-1cm. These limitations will ensure that the reflected beam of light has afocal point which is either virtual, i.e. associated with a divergingreflected beam, or at least 1 cm in front of the surface, such that therisk generated by the focus is significantly reduced. The surface mayalso have numerous regions with smaller curvatures, creating a diffusivesurface, as in FIG. 15C, which significantly helps reducing the risk ofa dangerous focal point. In such a case, the radius of curvature of eachsub section of the surface may be smaller than 1 cm without creating afocal point. Furthermore, if the surface is split into multiple zones,each zone may have smaller curvature.

In order to operate safely, the system also needs to be able to directthe power beam to the photovoltaic cell so that it is blocked by it, andnot be directed at some unsafe region. In order to accomplish that, adetector should be positioned to provide indication of the impingementof the beam on the receiver. Such a detector should typically bepositioned in the receiver, but configurations where such a detector islocated in the transmitter are also possible, in which case the detectorshould be responsive to a phenomenon occurring due to the impact of thebeam on the receiver. Such a transmitter-associated system may includeimage acquisition and processing of optical information received fromthe receiver, such as the reflection of the beam from a barcode printedon the receiver, so that the transmitter may detect the barcode'sillumination pattern. Reflections from a retro reflector or retroreflectors or arrays or patterns thereof may be positioned on thereceiver and such reflection may be detected in the transmitter, eitherby way of image processing, by measuring back reflection or by measuringcoherence effects of the reflection. The detector may be a current orvoltage sensor positioned in the receiver, a photodiode in the receiveror in the transmitter, or an imaging device which may be either in thetransmitter or the receiver. A retro-reflector in the vicinity of thephotovoltaic cell may also be used, in combination with an additionaldetector in the transmitter, detecting light reflected from theretroreflector.

The detector, upon detecting the beam of light impinging on thephotovoltaic cell, sends a signal accordingly to the system controller.If the detector is in the receiver, such signalling may be donewirelessly, using a communication channel which may be RF, IR, visiblelight, UV, modulation of the beam, TCP/IP, or sound. The systemcontroller is usually located in the transmitter, but may also belocated in a main control unit, which may even be on a computer networkfrom the transmitter. On receipt of the signal, the controller respondsby performing at least one of:

-   -   (a) Changing the state of the laser driver.    -   (b) Changing the operational properties of the beam steering        apparatus, such as the direction to which it directs the beam,        or the speed in which such direction is changed.

Reference is now made to FIG. 16, which is a schematic diagram showing adetailed description of the complete system. The system comprisestransmitter 21 and receiver 22. In general, the transmitter and receiverwill be located remotely from each other, but are shown in FIG. 16, forconvenience, to be close to each other. Beam 15 transfers power fromtransmitter 21 to receiver 22.

On the receiver 22, the front surface 7 reflects a small part ofincident beam 15 as a reflected beam 16, while either diffusing it orcreating a virtual focal point behind front surface 7, or a real focalpoint at least 1 cm in front of surface 7. After transmission throughthe at least partially transparent surface 7, beam 15 impinges on theoptical-to-electrical power converter 1.

The optical-to-electrical power converter 1 may be enclosed in a packagethat may have a front window, which may be surface 7 or a separatewindow. It may also be coated to have an external surface adapted tofunction as an interface with the air, or the adhesive or the glasssurrounding it. In a typical configuration, the optical-to-electricalpower converter 1 could be a junction of semiconductor layers, whichtypically have conductors deposited on them. In many embodiments surface7 would be coated on, or be the external surface of one of thesesemiconductor layers.

Signalling detector 8 indicates that beam 15 is impinging onphotovoltaic cell 1 and transmits that information to the controller 13,in this example system, located in the transmitter 21. The controlsignal is transmitted by a link 23 to a detector 24 on the transmitter.

Electrical power converter 1, has a bandgap E8 and typically yields avoltage between 0.35 and 1.1V, though the use of multi-junctionphotovoltaic cells may yield higher voltages. Power flows from thephotovoltaic cell 1 through conductors 2 a and 2 b, which have lowresistance, into inductor 3 which stores some of the energy flowingthrough it in a magnetic field.

Automatic switch 4, typically a MOSFET transistor connected to a controlcircuit (not shown in FIG. 16), switches between alternating states,allowing the current to flow through the inductor 3 to the ground for afirst portion of the time, and for a second portion of the time,allowing the inductor to emit its stored magnetic energy as a current ata higher voltage than that of the photovoltaic cell, through diode 5 andinto load 6, which can then use the power.

Automatic switch 4 may be operating at a fixed frequency or at variablefrequency and/or duty cycle and/or wave shape which may either becontrolled from the transmitter, or be controlled from the client load,or be based on the current, voltage, or temperature at the load, or bebased on the current, voltage or temperature at automatic switch 4, orbe based on the current, voltage or temperature emitted by theoptical-to-electrical power converter 1, or be based on some otherindicator as to the state of the system.

The receiver may be connected to the load 6 directly, as shown in FIG.16, or the load 6 can be external to the receiver, or it may even be aseparate device such as a cellphone or other power consuming device, andit may be connected using a socket such as USB/Micro USB/Lightning/USBtype C.

In most cases there would also be an energy storage device, such as acapacitor or a battery connected in parallel to load 6, or load 6 mayinclude an energy storage device such as a capacitor or a battery.

Transmitter 21 generates and directs beam 15 to the receiver 22. In afirst mode of operation, transmitter 21 seeks the presence of receivers22 either using a scanning beam, or by detecting the receiver usingcommunication means, such as RF, Light, IR light, UV light, or sound, orby using a camera to detect a visual indicator of the receivers, such asa retro-reflector, or retro-reflective structure, bar-code, highcontrast pattern or other visual indicator. When a coarse location isfound, the beam 15, typically at low power, scans the approximate areaaround receiver 22. During such a scan, the beam 15 impinges onphotovoltaic cell 1. When beam 15 impinges on photovoltaic cell 1,detector 8 detects it and signals controller 13 accordingly.

Controller 13 responds to such a signal by either or both of instructinglaser driver 12 to change the power P it inputs into gain medium 11 andor instructing mirror 14 to alter either its scan speed or direction itdirects the beam to or to hold its position, changing the scan stepspeed. When gain medium 11 receives a different power P from the laserpower supply 12, its small signal gain—the gain a single photonexperiences when it transverses the gain medium, and no other photonstraverse the gain medium at the same time,—changes. When a photon,directed in a direction between back mirror 10 and output coupler 9passes through gain medium 11, more photons are emitted in the samedirection—that of beam 15—and generate optical resonance between backmirror 10 and output coupler 9.

Output coupler 9 is a partially transmitting mirror, having reflectanceR, operating at least on part of the spectrum between the first overtoneof the C—H absorption at 6940 cm⁻¹ and the second overtone of the C—Habsorption at 8130 cm⁻¹, and is typically a multilayer dielectric orsemiconductor coating, in which alternating layers of differentrefractive index materials are deposited on a substrate, which istypically glass, plastic or the surface of gain medium 11. AlternativelyFresnel reflection can be used if the gain medium is capable ofproviding sufficient small signal gain or has a high enough refractiveindex, or regular metallic mirrors can be used. A Bragg reflector mayalso be used, should the gain medium be either a semiconductor or afibre amplifier. Output coupler 9 may also be composed of a highreflectance mirror combined with a beam extractor, such as asemi-transparent optical component that transmits a part of the lightand extracts another part of the light from the forward traveling waveinside the resonator, but typically also a third portion extracted fromthe backwards propagating wave inside the resonator.

Back reflector 10 should be a high reflectance mirror, although a smallamount of light may back-leak from it and may be used for monitoring orother purposes, working at least on part of the spectrum between thefirst overtone of the C—H absorption at 6940 cm⁻¹ and the secondovertone of the C—H absorption at 8130 cm⁻¹. It may typically beconstructed of alternating layers of different refractive indexmaterials deposited on a substrate, usually glass, metal or plastic.Alternatively Fresnel reflection can be used if the gain medium iscapable of providing sufficient small signal gain, or regular metallicmirrors can be used. A Bragg reflector may also be used should the gainmedium be either a semiconductor or a fibre amplifier.

Gain medium 11 amplifies radiation between the first overtone of the C—Habsorption at 6940 cm⁻¹ and the second overtone of the C—H absorption at8130 cm⁻¹, although not necessarily over the whole of this spectralrange. It is capable of delivering small signal gain larger than theloss caused by output coupler 9 when pumped with power P by laser driver12. Its area, field of view, and damage thresholds should be largeenough to maintain a beam of at least 8 kW/m²/Steradian/(1-R), where Ris the reflectance of output coupler 9. It may be constructed of eithera semiconductor material having a bandgap between 0.8-1.1 eV or of atransparent host material doped with Nd ions, or of another structurecapable of stimulated emission in that spectral range. Gain medium 11 ispositioned in the optical line of sight from the back reflector 10 tooutput coupler 9, thus allowing radiation reflected by the backreflector 10 to resonate between the back reflector 10 and the outputcoupler 9 through gain medium 11.

For the exemplary implementation where the gain medium 11 is asemiconductor having a bandgap between 0.8-1.1 eV, it should bepreferably attached to a heat extracting device, and may be pumpedeither electrically or optically by laser driver 12.

In the exemplary implementation where the gain medium 11 is atransparent host, such as YAG, YVO₄, GGG, or glass or ceramics, dopedwith Nd ions, then gain medium 11 should preferably also be in opticalcommunication with a filter for extracting radiation around 9400 cm⁻¹from that resonating between back mirror 10 and output coupler 9.

The beam steering apparatus 14 is shown controlled by controller 13. Itcan deflect beam 15 into a plurality of directions. Its area should belarge enough so that it would contain essentially most of beam 15 evenwhen tilted to its maximal operational tilt angle. Taking a simplistic2D example, if beam 15 were to be a collimated 5 mm diameter (1/e²diameter) Gaussian beam, and the beam steering apparatus were to be asingle round gimballed mirror centred on the beam centre, and if themaximal tilt required of the mirror is 30 degrees, and assuming thatbeam steering apparatus 14 has no other apertures, then if the mirrorhas a 5 mm diameter like that of the beam, it would have anapproximately 13% loss at normal incidence to the beam, butapproximately 60% loss at 60 degrees tilt angle. This would severelydamage the system's performance. This power loss is illustrated in thegraph of FIG. 17.

At the beginning of operation, controller 13 commands laser driver 12and mirror 14 to perform a seek operation. This may be done by aimingbeam 15, with the laser driver 12 operating in a first state, towardsthe general directions where a receiver 22 is likely to be found. Forexample, in the case of a transmitter mounted in a ceiling corner of aroom, the scan would be performed downwards and between the two adjacentwalls of the room. Should beam 15 hit a receiver 22 containing anoptical-to-electric power converter 1, then detector 8 would signal assuch to controller 13. So long as no such signal is received, controller13 commands beam steering 14 to continue directing beam 15 in otherdirections, searching for a receiver. If such a signal is received fromdetector 8, then controller 13 may command beam steering 14 to stop orslow down its scan to lock onto the receiver, and to instruct laserdriver 12 to increase its power emission. Alternatively controller 13may note the position of receiver 22 and return to it at a later stage.

When laser driver 12 increases its power emission, the small signal gainof gain medium 11 increases, and as a result beam 15 carries more powerand power transmission begins. Should detector 8 detect a power lossgreater than a threshold, which may be pre-determined or dynamicallyset, and which is typically at a level representing a significantportion of the maximal permissible exposure level, and which is alsotypically greater than the system noise figure, these conditionsimplying either that beam 15 is no longer aimed correctly at theoptical-to-electrical power converter 1, or that some object has enteredthe beam's path, or that a malfunction has happened, controller 13should normally command laser driver 12 to change its state, by reducingpower to maintain the required safety level. If another indication ofsafe operation is present, such as an indication from the user as to thesafety of transmission, which may be indicated by a user interface or anAPI, or an indication of safe operation from a second safety system, thecontroller may command the laser to increase power to compensate for thepower loss. The controller 13 may also command the beam steeringassembly 14 to perform a seeking operation again.

There may be two different stages in the seek operation. Firstly, acoarse search can be performed using a camera, which may search forvisual patterns, for a retro reflector, for high contrast images, for aresponse signal from receivers or for other indicators, or by using thescanning feature of beam steering 14. A list of potential positionswhere receivers may be found can thus be generated. The second stage isa fine seek, in which the beam steering mirror 14 directs beam 15 in asmaller area until detector 8 signals that beam 15 is impinging on anoptical-to-electrical power converter 1.

Reference is now made to FIG. 18, shows an example cooling system forthe gain medium 11 of the system of FIG. 16. Although the reflectors 9,10 are shown as separate optical elements, it is to be understood thatone or both of them may be coated directly on the gain medium end facesfor simplifying the system. Gain medium 11 converts the power itreceives from the laser driver 12 into both heat and photons, and wouldtypically degrade the system performance if the gain medium were to beheated above a certain temperature.

For that reason, gain medium 11 is attached to heatsink 34 using abonding agent 33 which is preferably a heat conducting solder having lowthermal resistance. Bonding agent 33 may also be a conductive adhesive.Bonding agent 33 may have a thermal expansion coefficient which isbetween that of gain medium 11 and heat sink 34. Heat sink 34 maytypically be a low thermal resistance heatsink made out of metal, whichmay be equipped with fins for increasing its surface area or an externalfluid pumping system such as a fan or a liquid pump 35.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

What is claimed is:
 1. A system for optical wireless power transmissionto a power receiving apparatus comprising: an optical resonator havingend reflectors and adapted to emit an optical beam; a gain mediumpositioned inside said optical resonator and having a first bandgapenergy, said gain medium being thermally attached to a cooling systemand configured to amplify light passing through it; a driver supplyingpower to said gain medium, and controlling the small signal gain of saidgain medium; a beam steering apparatus configured to direct said opticalbeam in at least one of a plurality of directions; anoptical-to-electrical power converter configured to convert said opticalbeam into electrical power having a voltage, said optical-to-electricalpower converter having a second bandgap energy; an electrical voltageconverter, adapted to convert the voltage of said electrical powergenerated by said optical-to-electrical power converter into a differentvoltage, said electrical power converter comprising an inductor, anenergy storage device and a switch; at least one surface associated withsaid optical-to-electrical power converter and optically disposedbetween said gain medium and said optical-to-electrical power converter,a detector configured to provide a signal indicative of said opticalbeam impinging on said optical-to-electrical power converter; and acontroller adapted to control at least one of the status of said beamsteering apparatus and said driver, said controller receiving a controlinput signal from at least said detector, wherein: said at least onesurface having properties such that it reflects a small part of lightincident on it, either (i) diffusively, or (ii) such that said reflectedlight has a virtual focus positioned remotely from said opticalresonator relative to said surface, or (iii) such that said reflectedlight has a real focus positioned at least 1 cm, in the direction ofsaid optical resonator relative to said surface; said controller isconfigured to respond to said control input signal received from saiddetector by at least one of: causing said driver to change the smallsignal gain of the gain medium; changing the radiance of said opticalbeam; changing the power supplied by said driver; changing the scanspeed of said beam steering apparatus; changing the scan position ofsaid beam steering apparatus; and recording a scan position defining theposition of said optical-to-electrical power converter; said gain mediumis a semiconductor device or a solid host doped with Nd ions, andincludes a filter attenuating radiation for at least one frequencyhaving a wave number in the range 8,300 cm-1 to 12,500 cm-1; said secondbandgap energy is smaller than said first bandgap energy; said firstbandgap energy is between 0.8 eV and 1.1 eV; said switch has a closedserial resistance smaller than R, given by the equation$R \leq \frac{E_{gain}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}$where R is measured in Ohms, Egain is the first bandgap energy measuredin Joules, and Plaser driver is the power supplied by the laser driverto the gain medium, measured in Watts, and said optical beam has aradiance of at least 8 kW/m2/Steradian, and a frequency between thefirst overtone of the C—H absorption situated at approximately 6940 cm-1and the second overtone of the C—H absorption situated at approximately8130 cm-1.
 2. A system according to claim 1 wherein said differentvoltage is a higher voltage than said voltage generated by saidoptical-to-electrical converter.
 3. A system according to claim 1wherein said status of said beam steering apparatus is at least one ofthe aiming direction or the scan speed of said beam steering apparatus.4. A system according to claim 1 wherein said optical beam has aradiance of at least 800 kW/m²/Steradian.
 5. A system according to claim1 wherein each one of said end reflectors of said resonator are either(i) dielectric mirrors, (ii) Bragg mirrors, (iii) Fresnel reflectors or(iv) mirrors composed of alternating layers of dielectric orsemiconductor material having different refractive indexes.
 6. A systemaccording to claim 1 wherein said gain medium is either a transparentsolid host material doped with Nd ions or a semiconductor.
 7. A systemaccording to claim 6, further comprising a filter for extractingradiation having a wave-number greater than 8300 cm⁻¹.
 8. A systemaccording to the semiconductor option of claim 6, wherein said gainmedium is a quantum dot gain medium.
 9. A system according to claim 1wherein said cooling system is at least one of a heatsink, a Peltierdiode, and a liquid cooled plate.
 10. A system according to claim 1wherein said cooling system is also equipped with a fan.
 11. A systemaccording to claim 9 wherein said gain medium is attached to saidcooling system using a layer of solder having less than 200° Kelvin/Wattthermal resistance.
 12. A system according to claim 9 wherein saidcooling system is such that the thermal resistance between the gainmedium and the surrounding air is less than 200° Kelvin/Watt.
 13. Asystem according to claim 1 wherein said optical-to-electrical powerconverter is a photovoltaic cell.
 14. A system according to claim 1wherein the serial resistance of said optical-to-electrical powerconverter is less than 1 Ohm.
 15. A system according to claim 13 whereinsaid photovoltaic cell is a III-V device.
 16. A system according toclaim 1 wherein said inductor has a serial resistance measured in Ohmsof less than the square of said first bandgap energy measured in Joulesdivided by 2*10⁻⁴⁰ times said driver power measured in Watts.
 17. Asystem according to claim 1 wherein said energy storage device is eithera capacitor or a rechargeable battery.
 18. A system according to claim 1further comprising a retro reflector.
 19. A system according to claim 1wherein said gain medium is pumped electrically or optically by saiddriver.
 20. A system according to claim 1 wherein said second bandgapenergy is more than 50% of said first bandgap energy.
 21. A method fortransmitting power from a transmitter to a receiver, comprising:converting a first electrical power to an electromagnetic wave having afrequency between the first overtone of the C—H absorption situated atapproximately 6940 cm-1 and the second overtone of the C—H absorptionsituated at approximately 8130 cm-1, said electromagnetic wave having aradiance of at least 8 kW/m2/Steradian, said converting being performedby using an optical resonator having end reflectors and a gain mediumconnected to a laser driver receiving said first electrical power, saidgain medium having a first bandgap energy between 0.8 eV and 1.1 eV,being positioned inside said optical resonator, being thermally attachedto a cooling system, and configured to amplify said electromagnetic wavepassing through it; directing said electromagnetic wave into at leastone of a plurality of directions using a beam steering apparatuscontrolled by a controlling unit; detecting the impingement of said beamon a target having an associated partially transparent surface, suchthat an indication relating to said impingement may be utilized by saidcontrolling unit to perform at least one of: causing a change in thesmall signal gain of the gain medium; causing a change in the radianceof said electromagnetic beam; causing a change in said first electricalpower; changing the scan speed of said beam steering apparatus; changingthe scan position of said beam steering apparatus; and recording a scanposition defining the position of said target; converting saidelectromagnetic wave into a second electrical power having a voltage, byusing an optical-to-electrical power converter having a second bandgapenergy smaller than said first bandgap energy; converting said voltageinto a different voltage using an electrical voltage converter,comprising an inductor, an energy storage device and a switch having aclosed serial resistance smaller than R, given by the equation${R \leq \frac{E_{gain}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}};$where R is measured in Ohms, Egain is the first bandgap energy measuredin Joules, and Plaser_driver is said first electrical power, measured inWatts, wherein said surface is designed such that it reflects a smallpart of said electromagnetic wave incident on it either (i) diffusively,or (ii) such that said reflected light has a virtual focus positionedremotely from said optical resonator relative to said surface, or (iii)such that said reflected light has a real focus positioned at least 1cm, in the direction of said optical resonator relative to said surface;and said gain medium is either a semiconductor device, or a solid hostdoped with Nd ions that includes a filter attenuating radiation for atleast one frequency having a wave number in the range 8,300 cm-1 to12,500 cm-1;
 22. The method in claim 21 wherein said switch is switchedat a frequency determined by the equations:$f < {\frac{1}{1.28*10^{- 40}*L}*E_{gain}^{2}*\frac{1 - \frac{E_{gain}}{5*10^{- 19}*V_{output}}}{P_{{laser}\_ {driver}}}}$$f > {\frac{1}{3*10^{- 38}*L}*E_{gain}^{2}*\frac{\left( {1 - \frac{E_{gain}}{4*10^{- 20}*V_{output}}} \right)}{P_{{laser}\_ {driver}}}}$where f is the switching frequency measured in Hz., Egain is the bandgapof the gain medium, measured in Joules, Voutput is the output voltagefrom the voltage converter, measured in Volts, and Plaser_driver is thepower pumped by the laser driver onto the gain medium, measured inWatts.
 23. The method of claim 21 wherein said detection of impingementof said beam on said target is done using either: detection in saidtransmitter of retro reflected illumination from said target, ordetection of illumination of said target using a receiver sensor. 24.The method of claim 21 wherein said second bandgap energy is more than50% of said first bandgap energy.